This invention relates to fuel cells in general and a method of managing the performance of a fuel cell in particular.
Fuel cells are electrochemical cells in which a free energy change resulting from an oxidation reaction is converted into electrical energy. A typical fuel cell consists of a fuel electrode (anode) and an oxidant electrode (cathode), separated by an ion-conducting electrolyte. The electrodes are connected electrically to a load (such as an electronic circuit) by an external circuit conductor. In the circuit conductor, electric current is transported by the flow of electrons, whereas in the electrolyte it is transported by the flow of ions, such as the hydrogen ion (H+) in acid electrolytes, or the hydroxyl ion (OHxe2x88x92) in alkaline electrolytes. A fuel (typically hydrogen) capable of chemical oxidation is supplied to the anode and ionizes on a suitable catalyst to produce ions and electrons. Gaseous hydrogen has become the fuel of choice for most applications, because of its high reactivity in the presence of suitable catalysts and because of its high energy density. Similarly, an oxidant (typically air) is supplied to the fuel cell cathode and is catalytically reduced. The most common oxidant is gaseous oxygen, which is readily and economically available from the air for fuel cells used in terrestrial applications. When gaseous hydrogen and oxygen are used as fuel and oxidant, the electrodes are porous to permit the gas-electrolyte junction to be as great as possible. The electrodes must be electronic conductors, and possess the appropriate reactivity to give significant reaction rates. Since the electrolyte is a non-electronic conductor, the electrons flow away from the anode via the external circuit. At the cathode, oxygen gas reacts with the hydrogen ions migrating through the electrolyte and the incoming electrons from the external circuit to produce water as a byproduct. The byproduct water is typically extracted as vapor. The overall reaction that takes place in the fuel cell is the sum of the anode and cathode reactions, with part of the free energy of reaction released directly as electrical energy. The difference between this available free energy and the heat of reaction is produced as heat.
In practice, a number of unit fuel cells are normally stacked or xe2x80x98gangedxe2x80x99 together in series to form a fuel cell assembly by abutting the anode current collector of one cell with the cathode current collector of its nearest neighbor in the stack. The fuel and oxidant are introduced through manifolds to their respective electrodes. In most traditional fuel cell applications the fuel and oxidant supply streams are designed as flow-through systems, however, these systems add a parasitic load to the fuel cell output and thus reduce the net power that can be extracted. In order to reduce the parasitic load, alternate configurations have been created in the prior art where the fuel stream or the oxidant stream or both are xe2x80x9cdead-endedxe2x80x9d. This dead-ended operation creates special problems such as water removal and accumulation of impurities.
In order to use fuel cells for portable applications such as radios or other portable consumer electronics, they need to be small and utilize air as the oxidant. Fuel cells for these applications are typically operated in a xe2x80x9cdead-endedxe2x80x9d fuel delivery system configuration with the cathode side open to air. A classical problem with air breathing fuel cells is water management. In theory, as the hydrogen in a dead-ended fuel cell is consumed to produce protons and electrons, the gas pressure is reduced and additional hydrogen is automatically introduced to equalize the unbalanced pressure gradient. Since the byproduct water is produced at the cathode (which is exposed to air), it evaporates away during normal operation. However, under heavy load, the evaporation rate lags the rate of formation and water tends to migrate back through the polymer electrolyte to the anode side. Some spots on a fuel cell are cooler than others, and the moisture condenses at these locations into liquid water, flooding the anode and impeding the reaction at the anode. Additionally, other impurities accumulate at the anode, and may poison the anode reaction sites. Inert contaminants also result in loss of performance by lowering the fuel partial pressure. In the prior art (see, for example, U.S. Pat. Nos. 5,366,818 and 4,537,839), these issues are addressed by a brief controlled release of the fuel gas at regular intervals. The purging operation involves controlled venting of a proportion (perhaps from 0.1 to 10%) of gaseous fuel or oxidant through a throttled opening. This purging action removes accumulated impurities, water and fine particulates from the anode side and restores fuel cell performance. Many schemes have also been taught in the prior-art to control the length of, and intervals between, successive purges, such as monitoring the fuel cell power output to provide for the exhaust to be approximately proportional to the amount of hydrogen consumed by the cell. However, release of hydrogen into the open air may create a safety hazard if the concentration of hydrogen is above four (4) percent by volume. It would be an advancement in the art of fuel cell systems to have a dead-ended system that can be purged without constituting a safety hazard.